Regulating the Electron Distribution of Metal‐Oxygen for Enhanced Oxygen Stability in Li‐rich Layered Cathodes

Abstract Li‐rich Mn‐based layered oxides (LLO) hold great promise as cathode materials for lithium‐ion batteries (LIBs) due to their unique oxygen redox (OR) chemistry, which enables additional capacity. However, the LLOs face challenges related to the instability of their OR process due to the weak transition metal (TM)‐oxygen bond, leading to oxygen loss and irreversible phase transition that results in severe capacity and voltage decay. Herein, a synergistic electronic regulation strategy of surface and interior structures to enhance oxygen stability is proposed. In the interior of the materials, the local electrons around TM and O atoms may be delocalized by surrounding Mo atoms, facilitating the formation of stronger TM─O bonds at high voltages. Besides, on the surface, the highly reactive O atoms with lone pairs of electrons are passivated by additional TM atoms, which provides a more stable TM─O framework. Hence, this strategy stabilizes the oxygen and hinders TM migration, which enhances the reversibility in structural evolution, leading to increased capacity and voltage retention. This work presents an efficient approach to enhance the performance of LLOs through surface‐to‐interior electronic structure modulation, while also contributing to a deeper understanding of their redox reaction.


Preparation of pristine material
The Li-rich Li1.2Ni0.2Mn0.6O2cathode materials were synthesized through molten salt assisted calcination method.Firstly, transition metal carbonate precursor was synthesized through carbonate precipitation method with a molar ratio of Mn: Ni = 0.6:0.2.An aqueous solution containing the required stoichiometric amounts of MnSO4 and NiSO4 was pumped into a 500 mL beaker with continuous stirring.2M Na2CO3 solution with a controlled amount of NH4OH as a chelating agent was simultaneously fed into the reactor.The temperature and pH of the solution were maintained at 55 °C and 8.0, respectively, throughout the coprecipitation reaction.The obtained Ni0.2Mn0.6(CO3)0.8precursor precipitate was filtered, washed, and dried in vacuum at 80 °C.Then, the precursor (0.927g), molten salt NaCl (1.87g), KCl (3.5784g) and lithium carbonate (0.4702g) (excess 5% to compensate for lithium loss under high temperature conditions) were ground and mixed uniformly.Afterwards, the mixture was transferred to the muffle furnace and calcined at 800 °C for 12 hours, and naturally cooled to room temperature.Finally, the pristine materials were washed by water, filtered, and dried at 180°C for 12 hours to obtain the final Li-rich Li1.2Ni0.2Mn0.6O2cathode (marked as P-LLO).

Preparation of Mo-treatment material
Firstly, the carbonate precursor was synthesized through the similar method as mentioned.Then, (NH4)6Mo7O24 • 7H2O (0.0530g), precursor (0.927g) and 25mL deionized water were added into the 50mL Teflon reaction vessel.After stirring 30 minutes, the Teflon reaction vessel was transferred to oven, kept at 150 o C for 12h.
Afterwards, the modified precursor was calcined with molten salt (NaCl, KCl) and lithium carbonate under the same procedure as the pristine materials.Finally, the modified materials were washed by water, filtered, and dried at 180°C for 12 hours to obtain the final Li-rich Li1.2Ni0.2Mn0.6O2cathode (marked as M-LLO).

Electrochemical measurements
The electrochemical performances of the samples were measured using a CR2030 type coin cell.The cathode materials were prepared as follows: 80 wt% of the synthesized powders, 10 wt% of carbon black, and 10 wt% of polyvinylidene fluoride (PVDF) were mixed using N-methyl pyrrolidinone (NMP) as a solvent.The resulting slurry was

Sample characterization
The atomic ratio of Li, Ni, Mn, and Mo were analyzed with ICPOES using Agilent 720ES.The surface structure and elemental distribution were measured by the scanning transmission electron microscopy (FEI Titan Cubed Themis G2300 STEM) equipped with a double-aberration corrector.High angle annular dark field (HAADF)-STEM imaging at atomic resolution (JEM-ARM200F) and energy dispersive X-ray spectroscopy (EDS) mapping were performed at 300 kV.In order to illustrate the changes of the lattice structure and local ligand geometry, powder XRD and pair distribution function (PDF) based on synchrotron total scattering data collection were conducted at the 11-ID-C beamline (λ =0.1173Å) of the Advanced Photon Source (APS), Argonne National Laboratory (ANL), U.S. The collected XRD patterns were refined based on the Rietveld method using Fullprof software.Besides, the scattering 2D patterns of the PDF were calibrated and integrated into 1D profiles by Fit 2D software, and then compute G(r) patterns through the Fourier transform.The refinement of the PDF profiles was conducted using PDFgui software based on the relevant structural model. [1]The soft X-ray absorption spectroscopy (SXAS) data of the Ni, Mn In situ synchrotron XRD characterizations were carried out using the 11-ID-C beamline (λ =0.1173Å) at the APS of Argonne National Laboratory.The adopted 2032 cell body was designed with two 3 mm holes on both sides of the cell and sealed with Kapton film for X-ray transmission.In situ cells were cycled at a constant current of 0.2C between 2.0-4.7 V vs. Li + /Li.The collected XRD patterns were calibrated and integrated by Fit 2D software, and then the refined based on the Lebail method through Fullprof software. [2]In situ Differential Electrochemical Mass Spectrometer (DEMS) measurement was carried out to detect the gas evolution during first cycle.The DEMS cell was assembled with a commercial Swagelok-type cell in an Ar-filled glovebox, where the diameter and loading density of electrode disc were 14 mm and 5 mg cm -2 , respectively.Ar carrier gas flowed through the measuring cell at a rate of 3.6 ml min -1 and then connected with the mass spectrometer (QAS 100).After ventilating for 6 hours until a stable baseline, cells were cycled between 2.0-4.7 V at a specific current of 40 mA/g on a Neware battery test system (CT-4008T-5V 50 mA-164).
All DFT calculations of P-LLO and M-LLO models were conducted on Vienna ab initio simulation package (VASP) with the Projector-Augmented-Wave (PAW) method and Perdew-Burke-Ernzerhof (PBE) of the Generalized Gradient Approximation (GGA) exchange-correlation functional. [3]The cut-off energy for all models was set to 520 eV and the convergence criteria were set to 1×10 -5 eV/cell of energy and 0.02 eV/Å of force, respectively.Spin-polarized calculation were considered in all tasks.To correct exchange correlation energy of TM atoms, the Hubbard U correction (GGA+U) method was applied where DFT+U values of Ni, Mn and Mo atoms were set to 6.2 eV, 3.9 eV, and 4.38 eV, respectively. [4]To simulate the TM migration process of the entire models, the ab initio molecular dynamics (AIMD) based on NVT ensemble at 300 K and 600 K were calculated, which can provide driving force for overcome migration energy barrier.
The bond strength can be analyzed by the Bader charge [5] and crystal orbital hamiltonian population (COHP). [6]r the surface model, according to the STEM result in Figure 1, models with 30 Å thickness were constructed to simulate experimental results with gradient properties.
Besides, the atomic composition of the P-LLO model and the M-LLO model was Li44Ni7Mn21O72 and Li22Ni29Mn21O72, respectively, which was based on the ICP (Table S1) and EDX (Figure S2).Meanwhile, 20 configurations including Li/Ni exchange and Ni/Mn exchange of the P-LLO model were considered, and the configuration with the lowest energy was selected.Besides, another Mo-doped surface model (Li44Ni6Mn21Mo1O72) was constructed to explore the effect of Mo atoms on the surface.
For the bulk models, the atomic composition of the P-LLO model and the M-LLO model was Li43Ni8Mn21O72 and Li43Ni7Mn21Mo1O72, respectively, which was based on the ICP (Table S1) results.Meanwhile, 20 configurations including Li/Ni exchange and Ni/Mn exchange of the P-LLO model were considered, and the configuration with the lowest energy was selected.To find the optimal Mo-doped site, the formation energies were calculated based on all possible substituted sites in the bulk Mo-LLO model.The formation energies (∆E form ) for different Mo-doped sites are described by the following equations.
Mo dopant at Mn site: Mo dopant at Ni site: Mo dopant at Li1 (Li in TM layer) and Li2 (Li in Li layer) sites: where  (−) is the total energy of the Mo-doped structure and  (𝐿𝐿𝑂) is bulk pristine LLO structure, respectively;  ( 3 ) ,  ( 2 ) ,  (𝑁𝑖𝑂) and  ( 2 ) are the total energies of bulk MoO3, MnO2, NiO and Li2O structures.To ensure that same amount of oxygen before and after reaction,  ( 2 ) is added to the equations.Note: According to previous report, [7] the O-O pair distance range from 2.5 Å to 3.      Hence, the valence of Mn can be calculated as 3.67+ according to the chemical composition of Ni-enrichment structure, which is in accordance with the Mn L-edge sXAS curve in Figure 3a.And then, there are 4.67 low-energy orbitals and two highenergy orbitals for the Mn 3.67+ atom, and two low-energy orbitals and two high-energy orbitals for the Ni 2+ atom, causing the ratio of high-energy and low-energy orbitals is equal to 1:0.713.Finally, the peak intensity at 532 eV is higher in the uncharged M-LLO sample than that of the P-LLO sample.
For the charged P-LLO sample (Li0.1Ni0.2Mn0.6O2), the Ni atoms are oxidized to 4+ and Mn atoms are difficult to oxidized leading to unchanged valence, as shown in the Figure Note2 and Table Note1.Therefore, the O atoms are oxidized to 1.65-to compensate the electron loss, resulting in a shoulder peak at 530.5 eV in Figure 3c.
Next, as mentioned, the Ni 4+ atom has two empty low-energy orbitals and two empty high-energy orbitals, while the Mn 4+ atom has 5 low-energy orbitals and 2 high-energy orbitals, causing the empty orbitals ratio is equal to 1:0.470.For the charged M-LLO sample (Li0.1Ni0.6Mn0.6O2), the Mn 3.67+ atoms are completely oxidized to 4+ during charging based on the result of Mn L-edge sXAS.Nevertheless, the valence of Ni is difficult to identified because the oxidization degree of O atoms is unknown.Here, we first assume that the O atoms are not oxidized, and then the valence of Ni atoms is calculated as 3.75+ by the chemical composition.Hence, the empty orbitals ratio is equal to 1:0.541 according to above discussion.Secondly, the valence of Ni should be lower than 3.75+, leading to the oxidized O atoms.Hence, the valence of Ni is set as the minimum of 2+ and the valence of O atoms can be calculated as 1.65-and the ratio of orbitals is calculated as 1:0.526, which is also larger than that of P-LLO.
In summary, the ratio of orbitals obtained from the above discussion well illustrate the variation of peak intensity ratios in the O K-edge sXAS curves.The peak intensity at coated on Al foil by a doctor blade technique and vacuum dried at 100 o C overnight.The cells were assembled in an Ar-filled glove box with the samples, lithium foil, a polymer separator and 1 M LiPF6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) (1: 1: 1 by volume) as the electrolyte.The chargedischarge cycling was performed in the potential window of 2.0-4.7 V (vs.Li/Li + ) at 28 o C with different current densities of 20 and 200 mA g -1 .

L
photoelectron spectroscopy (Thermo Scientific K-Alpha Nexsa).The binding energy values of the obtained spectra were calibrated using the C 1s peak at 284.8 eV.Neutron PDF experiments were performed on Multi-Physics Instrument (MPI) at China Spallation Neutron Source (CSNS), Dongguan, China.About 1g powder samples (pristine sample, carbon background and charged samples with carbon) were loaded into 9-mm quartz capillaries.The charged sample mixture was washed by dimethyl carbonate (DMC) to remove the binder and dried in a glovebox.After subtracting the signal of carbon based on the mass ratio, the obtained total scattering structure factor S(Q) data was further transformed into PDF G(r) data.

Figure S2 .
Figure S2.The XRD patterns and Rietveld refinement results of the (a) P-LLO and (b) M-LLO samples.

Figure S3 .
Figure S3.Pair distribution function (PDF) and refinement results patterns by the PDFgui analysis on (a) P-LLO and (b) M-LLO.

Figure S4 .
Figure S4.(a, b) Initial charge/discharge profiles at 0.1C rate for P-LLO and M-LLO samples between 2.0-4.7 V and (c, d) The corresponding dQ/dV curves of samples.

Figure S5 .
Figure S5.O 1s XPS spectra of the P-LLO and M-LLO sample at initial states.

Figure S7 .
Figure S7.The lithiated and de-lithiated Mo-doped LLO (Mo-LixNi0.2Mn0.6O2,where x = 0.4 and 0.2) surface models.The orange arrows illustrate the model from left to right representing bulk to surface.

Figure S9 .
Figure S9.Net average Bader charge of Mn and Ni atoms in P-LLO and M-LLO models in the initial and charged state, where the minus sign represents the electron loss.

Figure S10 .
Figure S10.Formation energies of Mo-LLO models at different substituted sites.The inset shows the specific Mo-doped sites.The purple octahedrons, green spheres/octahedrons, grey octahedron, and blue octahedron represent the Mn octahedron, Li atoms /octahedrons, Ni octahedron, and Mo octahedron, respectively.

Figure S13 .
Figure S13.Net average Bader charge of Mn and Ni atoms in P-LLO and M-LLO models during the de-lithiation process, where the minus sign represents the electron loss.

Figure S14 .
Figure S14.Net average Bader charge of Nireplaced and Mo atoms in P-LLO and M-LLO models during the de-lithiation process, where the minus sign represents the electron loss and Nireplaced represents this Ni site in P-LLO model is the same as Mo site in M-LLO model.

Figure S15 .
Figure S15.Comparison of the ex situ neutron PDF results of P-LLO and M-LLO collected at initial and charged states.
2 Å of two samples at initial state is ascribe to the interlayer and intralayer O-O in TM-O octahedra, where the main peak is about 2.76 Å.For charged sample, the large amount of short O-O pairs appear at 2.3 Å-2.6 Å, which is due to the oxygen redox reaction that makes the interlayer O-O contraction.Moreover, the peak position of short O-O pair for the M-LLO sample is larger than that of the P-LLO sample, indicating that the short O-O pair distance in M-LLO sample is increased compared to that of P-LLO at high voltages.

Figure S16 .
Figure S16.The valence electron differential charge density and Bader charge (red circles) for bulk P-LLO (a) and M-LLO (b) models, where the iso-surface is set to 0.035 e/Å 3 and the positive sign represents the obtained electrons.

Figure S17 .
Figure S17.Electron localized function (ELF) diagram of the P-LLO model (a) and the M-LLO model (b), where ELF values of 0, and 1 represent completely delocalized electrons and completely localized electrons, respectively.In this ELF diagram, ELF = 0.3 is set to the maximum saturation for easy comparison.

Figure S18 .
Figure S18.The structure of bulk P-LLO (a) and M-LLO (b) models after AIMD process at 600 K and NVT ensemble.
segregation structure should be determined first.Because the detective limits of sXAS is about 10 nm depth, the atomic ratio of Ni and Mn in the whole 10 nm depth can be obtained from STEM-EDS results, and the corresponding surface chemical composition of M-LLO sample with a 10 nm thickness is Li1.0Ni0.4Mn0.6O2shown in the FigureNote1.Next, the valence of Ni can be determined as 2+ since the Ni L-edge sXAS curve of the uncharged M-LLO sample is the same as that of the P-LLO sample.
532 eV is highly related to the valence of TM and Ni content.Since the change of the valence of TM is still induced by the Ni-enrichment structure in M-LLO.Here, we think the enhanced peak intensity at 532 eV for the M-LLO sample is finally ascribed to the Ni-enrichment structure.

Figure Note2 .
Figure Note2.The electronic arrangement of TM-O hybrid orbitals (eg and t2g) for the charged P-LLO and M-LLO samples, where Mn is the high-spin system and Ni is the low-spin system.

Table S1 .
The element distribution obtained from ICP results and STEM-EDS results.The data obtained by STEM-EDX is the atomic ratio of each element.We use Ni as 0.2 to convert the corresponding composition of Mn and Mo in the composition formula. *

Table Note2
The atomic ratio, average valence of Ni and Mn atoms, empty orbitals number, and the ratio of orbitals in the charged and uncharged P-LLO and M-LLO samples with 10 nm depth.